BAW resonator bi-layer top electrode with zero etch undercut

ABSTRACT

A piezoelectric resonator includes a multi-layer top electrode configured such that a top most layer protects the underlying layers from subsequent etching, thereby preventing etch undercut of the top-most layer. In one embodiment, the multi-layer top electrode is configured as a bi-layer, so that the upper layer of the bi-layer stack protects all sides of the underlying layer from subsequent etch process steps. In an alternative embodiment, at least the perimeter of a multi-layer top electrode is completely covered with overlapping interconnect metal.

RELATED APPLICATION(S)

This Application is a divisional application of U.S. patent applicationSer. No. 11/442,375, filed on May 25, 2005 now U.S. Pat. No. 7,600,303,and entitled “A BAW Resonator Bi-Layer Top Electrode With Zero EtchUndercut.” The U.S. patent application Ser. No. 11/442,375, filed on May25, 2005, and entitled “A BAW Resonator Bi-Layer Top Electrode With ZeroEtch Undercut,” is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the field of piezoelectric resonators.More particularly, the present invention relates to the field of a bulkacoustic wave (BAW) resonator with a bi-layer top electrode with zeroetch undercut.

BACKGROUND OF THE INVENTION

Piezoelectric resonators are primarily used in RF filters andoscillators. RF filters are increasingly being used in mobilecommunications devices. These resonators are commonly referred to asbulk acoustic wave (BAW) resonators. Other acronyms for the same orsimilar devices include FBAR (thin-film bulk acoustic resonators), SMR(solidly mounted resonators), TFR (thin film resonators), or SCR(stacked crystal resonators). The resonators are interconnected,typically at the upper metal level, to build RF filters.

It is known that a bulk acoustic wave (BAW) resonator in generalcomprises a piezoelectric layer sandwiched between two electronicallyconductive layers that serve as electrodes. When a radio frequency (RF)signal is applied across the device, it produces a mechanical wave inthe piezoelectric layer. BAW resonators are typically fabricated onsilicon (Si), gallium arsenide (GaAs), glass, or ceramic substrates. BAWresonators are typically manufactured using various thin filmmanufacturing techniques, such as for example sputtering, vacuumevaporation or chemical vapor deposition. BAW resonators utilize apiezoelectric thin film layer for generating the acoustic bulk waves.The resonance frequencies of typical BAW resonators range from 0.5 GHzto 5 GHz, depending on the size and materials of the device. BAWresonators exhibit the typical series and parallel resonances of crystalresonators. The resonance frequencies are determined mainly by thematerial of the resonator and the dimensions of the layers of theresonator.

A typical BAW resonator consists of an acoustically active piezoelectriclayer, electrodes on opposite sides of the piezoelectric layer, and anacoustical isolation from the substrate. Although the resonant frequencyof a BAW device also depends on other factors, the thickness of thepiezoelectric layer is the predominant factor in determining theresonant frequency. As the thickness of the piezoelectric layer isreduced, the resonance frequency is increased.

The material used to form the electrode layers is an electricallyconductive material. The acoustical isolation is produced with asubstrate via-hole, a micromechanical bridge structure, or with anacoustic mirror structure. In the via-hole and bridge structures, theacoustically reflecting surfaces are the air interfaces below and abovethe devices. The bridge structure is typically manufactured using asacrificial layer, which is etched away to produce a free-standingstructure. Use of a sacrificial layer makes it possible to use a widevariety of substrate materials, since the substrate does not need muchmodification, as in the via-hole structure. A bridge structure can alsobe produced using an etch pit structure, in which case a pit is etchedin the substrate or the material layer below the BAW resonator toproduce the free standing bridge structure.

FIG. 1 illustrates a cross-section of a conventional piezoelectricresonator. The piezoelectric resonator includes a substrate 10, anacoustic mirror or acoustic reflector 20, a bottom electrode 30, apiezoelectric layer 40, and a top electrode 50. The top electrode can beconstructed from several metallic and dielectric layers. The bottomelectrode 30 can also be constructed from several metallic anddielectric layers; molybdenum is typically used. The top electrode 50shown in FIG. 1 includes a bottom metal layer 52 of a material with highacoustic impedance, and a top metal layer 54 of a material with lowimpedance.

FIGS. 2-5 illustrate conventional fabrication steps of the top electrode50. As illustrated in FIG. 2, the bottom metal layer 52 is deposited onthe piezoelectric layer 40. The bottom metal layer 52 is then patternedand etched, as illustrated in FIG. 3. FIG. 4 illustrates the top metallayer 54 deposited on the etched bottom metal layer 52. The top metallayer 54 is then patterned and etched, as illustrated in FIG. 5. Thebottom metal layer 52 is exposed during the etch step of top metal layer54. As a result, a portion 56 of the bottom metal layer 52 is etched, orremoved, while etching the top metal layer 54. Additionally, when aninterconnect metal layer is fabricated to make contact with the topelectrode 50, further undercutting of the top layer 54 can occur duringan etch step of the interconnect metal layer. In either case,undercutting of the top metal layer 54 negatively impacts theperformance of the piezoelectric resonator.

During fabrication of conventional piezoelectric resonators, such as theprocess illustrated in FIGS. 2-5, problems arise during etching of thebi-layer top electrode and during etching of the metallization layerthat contacts the top electrode bi-layer stack. The difficulty isfinding etches that do not preferentially attack the bottom metal layer52, which can be molybdenum, causing undercut voiding at the deviceperiphery. This is particularly true if wet etching is employed becauseof various galvanic and catalytic reactions that do occur.

SUMMARY OF THE INVENTION

Embodiments of a piezoelectric resonator include a multi-layer topelectrode. The multi-layer top electrode is configured such that a topmost layer protects the underlying layers from subsequent etching,thereby preventing etch undercut of the top electrode. Preventing etchundercut provides a superior piezoelectric resonator architecturebecause of good control of the active area in-plane dimensions. In oneembodiment, the multi-layer top electrode is configured as a bi-layer,so that the upper layer of the bi-layer stack protects all sides of theunderlying layer from subsequent etch process steps.

Standard IC fabrication methods are used for the basic manufacturingsequences, including depositions, photolithography, and etch processes.MEMS techniques can also be employed for packaging and resonatoracoustic isolation from the substrate.

The multi-layer top electrode construction is desirable in BAW resonatordesign that require stringent acoustic, electrical, and processintegration requirements. By using two distinct materials for themulti-layer configuration, it is possible to benefit from the uniqueproperties of each material.

In an alternative embodiment, at least the perimeter of a multi-layertop electrode is completely covered with overlapping interconnect metal.This alternative approach has the advantage of eliminating a photo step,for example the top-most layer patterning step, but it has thedisadvantage that it only protects the top electrode edges during theinterconnect etch and not during the top electrode etch.

In one aspect of the present invention, a piezoelectric resonatorincludes a bottom electrode, a piezoelectric layer coupled to the bottomelectrode, and a top electrode coupled to the piezoelectric layer,wherein the top electrode comprises a bottom metal layer and a top metallayer configured such that the top metal layer isolates the bottom metallayer from subsequent etch steps. The bottom metal layer can comprise arefractory metal. The bottom metal layer can comprise molybdenum. Thebottom metal layer can also comprise ruthenium, tungsten, platinum,osmium, rhenium or iridium. The top metal layer can comprise aluminum,gold, platinum, or an aluminum alloy composition. The top metal layercan also comprise a metal alloy composition. The piezoelectric resonatorcan also include an interconnect metal layer coupled to the top metallayer. The interconnect metal layer is physically isolated from thebottom metal layer of the top electrode. The interconnect metal layercan comprise titanium tungsten, tungsten, or molybdenum. Theinterconnect metal layer can comprise a first interconnect metal layerand a second interconnect metal layer. The first interconnect metallayer can comprise titanium tungsten. The second interconnect metallayer can comprise aluminum copper or copper. The interconnect metallayer can comprise a metal material that is selectively etched relativeto a material of the top metal layer of the top electrode. Thepiezoelectric resonator can be a bulk acoustic wave resonator. The topmetal layer can completely overlap the bottom metal layer.

In another aspect of the present invention, a method of fabricating apiezoelectric resonator is described. The method includes providing abottom electrode and a piezoelectric layer coupled to the bottomelectrode, depositing a bottom metal layer of a top electrode on thepiezoelectric layer, patterning and etching the bottom metal layer,depositing a top metal layer of the top electrode on the etched bottommetal layer, and patterning and etching the top metal layer such thatthe top metal layer completely overlaps the bottom metal layer. Themethod can also include depositing an interconnect metal layer on thetop metal layer, and patterning and etching the interconnect metal layersuch that the bottom metal layer of the top electrode is isolated fromthe etching. The interconnect metal layer is physically isolated fromthe bottom metal layer of the top electrode. The interconnect metallayer can comprise a first interconnect metal layer and a secondinterconnect metal layer. The first interconnect metal layer cancomprise titanium tungsten. The second interconnect metal layer cancomprise aluminum copper or copper. The interconnect metal layer cancomprise titanium tungsten, tungsten, or molybdenum. The interconnectmetal layer can comprise a metal material that is selectively etchedrelative to a material of the top metal layer of the top electrode. Thepiezoelectric resonator can be a bulk acoustic wave resonator. The topmetal layer can overlap the bottom metal layer by about 0.3 um to about3 um. The bottom metal layer can comprise a refractory metal. The bottommetal layer can comprise molybdenum. The bottom metal layer can compriseruthenium, tungsten, platinum, osmium, rhenium or iridium. The top metallayer can comprise aluminum, gold, platinum, or an aluminum alloycomposition. The top metal layer can comprise a metal alloy composition.

In yet another aspect of the present invention, a piezoelectricresonator includes a bottom electrode, a piezoelectric layer coupled tothe bottom electrode, a top electrode coupled to the piezoelectriclayer, wherein the top electrode comprises a bottom metal layer and atop metal layer, and an interconnect metal layer coupled to the topelectrode and configured such that the metal interconnect layer isolatesthe bottom metal layer from a metal interconnect etch step. Theinterconnect metal layer can be configured to cover a perimeter of thetop electrode such that the bottom metal layer is isolated. The bottommetal layer can comprise a refractory metal. The bottom metal layer cancomprise molybdenum. The bottom metal layer can comprise ruthenium,tungsten, platinum, osmium, rhenium or iridium. The top metal layercomprises aluminum, gold, platinum, or an aluminum alloy composition.The top metal layer can comprise a metal alloy composition. Theinterconnect metal layer can be configured to be in physical contactwith the top metal layer and the bottom metal layer of the topelectrode. The interconnect metal layer can comprise a firstinterconnect metal layer and a second interconnect metal layer. Thefirst interconnect metal layer can comprise titanium tungsten. Thesecond interconnect metal layer can comprise aluminum copper or copper.The interconnect metal layer can comprise titanium tungsten, tungsten,or molybdenum. The interconnect metal layer can comprise a metalmaterial that is selectively etched relative to a material of the topmetal layer of the top electrode. The piezoelectric resonator can be abulk acoustic wave resonator.

In another aspect of the present invention, another method offabricating a piezoelectric resonator is described. The method includesproviding a bottom electrode and a piezoelectric layer coupled to thebottom electrode, depositing a bottom metal layer of a top electrode onthe piezoelectric layer, patterning and etching the bottom metal layer,depositing a top metal layer of the top electrode on the etched bottommetal layer, patterning and etching the top metal layer, depositing aninterconnect metal layer on the etched top metal layer such that theinterconnect metal layer isolates the bottom metal layer from subsequentetch steps. The method can also include patterning and etching theinterconnect metal layer such that the bottom metal layer of the topelectrode is isolated from the etching. The interconnect metal layer canbe configured to cover a perimeter of the top electrode such that thebottom metal layer is isolated. Depositing the interconnect metal layercouples the interconnect metal layer in physical contact with the topmetal layer and the bottom metal layer of the top electrode. Theinterconnect metal layer can comprise a first interconnect metal layerand a second interconnect metal layer. The first interconnect metallayer can comprise titanium tungsten. The second interconnect metallayer can comprise aluminum copper or copper. The interconnect metallayer can comprise titanium tungsten, tungsten, or molybdenum. Theinterconnect metal layer can comprise a metal material that isselectively etched relative to a material of the top metal layer of thetop electrode. The piezoelectric resonator can be a bulk acoustic waveresonator. The bottom metal layer can comprise a refractory metal. Thebottom metal layer can comprise molybdenum. The bottom metal layer cancomprise ruthenium, tungsten, platinum, osmium, rhenium or iridium. Thetop metal layer can comprise aluminum, gold, platinum, or an aluminumalloy composition. The top metal layer can comprise a metal alloycomposition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-section side view of a conventionalpiezoelectric resonator.

FIGS. 2-5 illustrate exemplary fabrication steps used to fabricate thetop electrode of the piezoelectric resonator in FIG. 1.

FIG. 6 illustrates a cross-section side view of a first embodiment ofthe piezoelectric resonator.

FIGS. 7-10 illustrate exemplary fabrication steps used to fabricate thetop electrode of the piezoelectric resonator in FIG. 6.

FIG. 11 illustrates an interconnect metal layer added to thepiezoelectric resonator of FIG. 10.

FIG. 12 illustrates the results after a subsequent etch of aninterconnect metal layer on the conventional piezoelectric resonator ofFIG. 5.

FIG. 13 illustrates an alternative embodiment of the piezoelectricresonator.

Embodiments of the piezoelectric resonator are described relative to theseveral views of the drawings. Where appropriate and only whereidentical elements are disclosed and shown in more than one drawing, thesame reference numeral will be used to represent such identicalelements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 6 illustrates a cross-section side view of a first embodiment ofthe piezoelectric resonator. The piezoelectric resonator includes anacoustic mirror or acoustic reflector 120 deposited on a substrate 110.A bottom electrode 130 is deposited on the acoustic reflector 120. Apiezoelectric layer 140 is deposited on the bottom electrode 130. A topelectrode 150 is deposited on the piezoelectric layer 140. In this firstembodiment, the top electrode 150 is configured as a bi-layer, includinga bottom metal layer 152 enclosed by a top metal layer 154. The topmetal layer 154 seals the edges of the bottom metal layer 152 preventingany edge etch of the bottom metal layer 152. The bottom metal layer 152is made of molybdenum and the top metal layer 154 is made of aluminum.Molybdenum is a preferred material for the bottom metal layer 152because of its acoustic properties. Alternative materials for the bottommetal layer 152 include refractory metals such as ruthenium, tungsten,platinum, osmium, iridium, or rhenium. Aluminum is preferred for the topmetal layer 154 because of its high electrical conductivity and for itsability to act as an etch stop during subsequent etch process steps.Alternative materials for the top metal layer 154 include an aluminumalloy, gold, platinum, or a specific alloy composition that has a highselectivity from subsequent etch steps.

FIGS. 7-10 illustrate exemplary fabrication steps used to fabricate thetop electrode of the piezoelectric resonator in FIG. 6. As illustratedin FIG. 7, the bottom metal layer 152 is deposited on the piezoelectriclayer 140. The bottom metal layer 152 is then patterned and etched, asillustrated in FIG. 8. FIG. 9 illustrates the top metal layer 154deposited on the etched bottom metal layer 152. The top metal layer 154is then patterned and etched, as illustrated in FIG. 9. The top metallayer 154 is patterned and etched to overlap the bottom metal layer 152on all sides. The overlap is tailored to optimize both processintegration and resonator performance. In one embodiment, about 0.3 umto about 3 um of overlap is employed. The overlap prevents etch undercutof the top metal layer 152.

FIG. 11 illustrates an interconnect metal layer 160 added to thepiezoelectric resonator of FIG. 10. The interconnect metal layer 160 isfabricated by depositing a metal layer on the top electrode 150 and thepiezoelectric layer 140, patterning, and etching the deposited metallayer. The interconnect metal layer 160 is titanium tungsten (TiW).Alternatively, the interconnect metal layer 160 is tungsten, molybdenum,or any material that is selectively etched relative to the top metallayer 154 of the top electrode 150. For example, a TiW interconnectmetal layer 160 is removed selectively to an aluminum top metal layer154 by a peroxide based wet etch. The interconnect metal layer 160 canalso consist of a bi-layer, for example TiW/AlCu or TiW/Cu. In thisexample, the AlCu or the Cu is for low resistance interconnect, whilethe TiW can be etched with fine selectivity to the top electrode 150.The piezoelectric resonators shown in FIGS. 6 and 11 allow for thestacking of films to create a top electrode that prevents any etchundercut. The interconnect metal layer 160 can be used to build a filterout of a fixed number of resonators.

For comparative purposes, FIG. 12 illustrates the results after asubsequent etch of an interconnect metal layer 60 on the conventionalpiezoelectric resonator of FIG. 5. As is shown in FIG. 12, etching theinterconnect metal layer 60 further undercuts the top metal layer 54 byremoving an additional portion 58 of the exposed bottom metal layer 52.In contrast, the bottom metal layer 152 of the piezoelectric resonatorin FIG. 11 is completely isolated from etching of the interconnect metallayer 160, as well as from the original etching of the top metal layer154.

An alternative embodiment of the piezoelectric resonator includes a topmetal layer of a top electrode patterned and etched to match a bottommetal layer of the top electrode, and an interconnect metal layerpatterned to cover the edges of the top electrode. In this manner, thebottom metal layer is protected during etching of the interconnect metallayer. FIG. 13 illustrates the alternative embodiment of thepiezoelectric resonator. Resonator 1 and resonator 2 are both fabricatedaccording to the fabrication steps used related to FIGS. 2-5. As such, aportion 56 of each bottom metal layer 52 is removed due to etch undercutduring etching of the top metal layers 54. After the top metal layer 54is etched, the interconnect metal layer 260 is deposited. Theinterconnect metal layer 260 is then patterned such that all edges ofthe underlying top electrode, including the top metal layer 54 and thebottom metal layer 54, remain covered by the interconnect metal layer260 after a subsequent etch step. After patterning, the interconnectmetal layer 260 is etched. Since the interconnect metal layer 260 thatcovers the edges of the top electrode are not etched, etch undercut ofthe top metal layer 54 is prevented during etching of the interconnectmetal layer 260. In this manner the second embodiment of thepiezoelectric resonator prevents etch undercutting while etching theinterconnect metal layer.

The piezoelectric resonator fabrication methods described above can beoptimized to reduce lateral spurious modes in the resonator devicewithout additional processing steps. Spurious modes are reduced byuniquely tailored resonator loading at the resonator perimeter. Examplesof such resonator loading techniques are described in U.S. Pat. No.6,812,619, which is hereby incorporated by reference.

It is understood by those skilled in the art that the terms depositing,patterning, and etching used above are intended as general descriptiveterms used in the fabrication processes. The fabrication steps describedabove can be performed using any conventional fabrication methodscapable of depositing, patterning, and etching the layers described.

The piezoelectric resonators are used as reference oscillators,stand-alone filters, and also as building blocks for RF filters. Such RFfilters can be used to replace SAW devices. The piezoelectric devicescan also be used in all RF stages of products needing filtering, eitherband filtering, or channel filtering.

The present invention has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the invention. Suchreferences, herein, to specific embodiments and details thereof are notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications can be made inthe embodiments chosen for illustration without departing from thespirit and scope of the invention.

1. A piezoelectric resonator comprising: a. a bottom electrode; b. apiezoelectric layer coupled to the bottom electrode; and c. a topelectrode coupled to the piezoelectric layer, wherein the top electrodecomprises a bottom metal layer and a top metal layer configured suchthat the top metal layer isolates the bottom metal layer from subsequentetch steps.
 2. The piezoelectric resonator of claim 1 wherein the bottommetal layer comprises a refractory metal.
 3. The piezoelectric resonatorof claim 2 wherein the bottom metal layer comprises molybdenum.
 4. Thepiezoelectric resonator of claim 2 wherein the bottom metal layercomprises ruthenium, tungsten, platinum, osmium, rhenium or iridium. 5.The piezoelectric resonator of claim 1 wherein the top metal layercomprises aluminum, gold, platinum, or an aluminum alloy composition. 6.The piezoelectric resonator of claim 1 wherein the top metal layercomprises a metal alloy composition.
 7. The piezoelectric resonator ofclaim 1 further comprising an interconnect metal layer coupled to thetop metal layer.
 8. The piezoelectric resonator of claim 7 wherein theinterconnect metal layer is physically isolated from the bottom metallayer of the top electrode.
 9. The piezoelectric resonator of claim 7wherein the interconnect metal layer comprises a first interconnectmetal layer and a second interconnect metal layer.
 10. The piezoelectricresonator of claim 9 wherein the first interconnect metal layercomprises titanium tungsten.
 11. The piezoelectric resonator of claim 9wherein the second interconnect metal layer comprises aluminum copper orcopper.
 12. The piezoelectric resonator of claim 7 wherein theinterconnect metal layer comprises titanium tungsten, tungsten, ormolybdenum.
 13. The piezoelectric resonator of claim 7 wherein theinterconnect metal layer comprises a metal material that is selectivelyetched relative to a material of the top metal layer of the topelectrode.
 14. The piezoelectric resonator of claim 1 wherein thepiezoelectric resonator comprises a bulk acoustic wave resonator. 15.The piezoelectric resonator of claim 1 wherein: the bottom metal layercomprises a top surface, a bottom surface opposite the top surface, anda sidewall surface disposed around the perimeter of the bottom metallayer between the top surface and the bottom surface; and the top metallayer completely overlaps the bottom metal layer, wherein the top metallayer covers the entire sidewall surface of the bottom metal layer. 16.A piezoelectric resonator comprising: a. a bottom electrode; b. apiezoelectric layer coupled to the bottom electrode; c. a top electrodecoupled to the piezoelectric layer, wherein the top electrode comprisesa bottom metal layer and a top metal layer; and d. an interconnect metallayer coupled to the top electrode and configured such that the metalinterconnect layer isolates the bottom metal layer from a metalinterconnect etch step.
 17. The piezoelectric resonator of claim 16wherein the interconnect metal layer is configured to cover a perimeterof the top electrode such that the bottom metal layer is isolated. 18.The piezoelectric resonator of claim 16 wherein the bottom metal layercomprises a refractory metal.
 19. The piezoelectric resonator of claim18 wherein the bottom metal layer comprises molybdenum.
 20. Thepiezoelectric resonator of claim 18 wherein the bottom metal layercomprises ruthenium, tungsten, platinum, osmium, rhenium or iridium. 21.The piezoelectric resonator of claim 16 wherein the top metal layercomprises aluminum, gold, platinum, or an aluminum alloy composition.22. The piezoelectric resonator of claim 16 wherein the top metal layercomprises a metal alloy composition.
 23. The piezoelectric resonator ofclaim 16 wherein the interconnect metal layer is configured to be inphysical contact with the top metal layer and the bottom metal layer ofthe top electrode.
 24. The piezoelectric resonator of claim 16 whereinthe interconnect metal layer comprises a first interconnect metal layerand a second interconnect metal layer.
 25. The piezoelectric resonatorof claim 24 wherein the first interconnect metal layer comprisestitanium tungsten.
 26. The piezoelectric resonator of claim 24 whereinthe second interconnect metal layer comprises aluminum copper or copper.27. The piezoelectric resonator of claim 16 wherein the interconnectmetal layer comprises titanium tungsten, tungsten, or molybdenum. 28.The piezoelectric resonator of claim 16 wherein the interconnect metallayer comprises a metal material that is selectively etched relative toa material of the top metal layer of the top electrode.
 29. Thepiezoelectric resonator of claim 16 wherein the piezoelectric resonatorcomprises a bulk acoustic wave resonator.
 30. The piezoelectricresonator of claim 16 wherein: the bottom metal layer comprises a topsurface, a bottom surface opposite the top surface, and a sidewallsurface disposed around the perimeter of the bottom metal layer betweenthe top surface and the bottom surface; and the interconnect metal layercovers the entire sidewall surface of the bottom metal layer.